In conventional gas-operated engines, gaseous fuel, such as LP fuel or natural gas, is fed from a source under pressure through a pressure regulator/vaporizer to a mixer or carburetor. The pressure regulator/vaporizer functions to feed gas vapor at constant pressure to the mixer. The mixer functions to mix the gaseous fuel with air, and to feed the mixture to the engine intake manifold for transmission to the cylinder intake ports. It is important in fuel delivery systems of this character to provide a mechanism for terminating fuel flow when the engine ceases operation to help insure that fuel vapor will not flow to the hot engine and potentially cause a combustible mixture to form in the exhaust manifold.
It is a general object of the present invention to provide a fuel control system and method for gas-operated engines in which fuel flow is affirmatively terminated when engine operation ceases. Another object of the present invention is to provide a fuel control system and method for gas-operated engines that obtain improved control of air/fuel ratio during all phases of engine operation.
A fuel control system for delivering gaseous fuel from a source to a gas-operated engine in accordance with one aspect of the present invention includes a normally open electronic control valve for connection between the fuel source and the engine, and responsive to electronic valve control signals for variably closing connection between the source and the engine. An electronic control unit, for supplying the electronic signals responsive to engine operation, includes facility responsive to termination of engine operation for automatically generating a valve control signal to hold the valve fully closed for a preselected time duration. This preselected time duration preferably includes a fixed minimum time duration and a user-programmable additional valve closure time duration.
In accordance with another aspect of the present invention, the system includes an oxygen sensor for operative coupling to the engine to supply a signal indicative of air/fuel ratio around a stoichiometric value, and at least one additional sensor for supplying an electrical sensor signal as a function of engine operation. The electronic control unit includes an electronic memory in which a table of base valve control signals is stored. The control unit is responsive to electrical signals from the at least one additional sensor for obtaining a corresponding base control signal from the table. The control unit is also responsive to a signal from the oxygen sensor for modifying the base control signals as a function of air/fuel ratio at the engine, and applying the modified control signals to the electronic control valve. The control unit is also responsive to absence of a usable signal from the oxygen sensor for providing the valve control signal in a predetermined manner independent of the table. In the preferred embodiment of the invention, the electronic control unit is responsive to absence of a usable signal from the oxygen sensor upon initial start-up, when the engine has not been operating for a sufficient time for the oxygen sensor to reach operating temperature, and in the event of malfunction or severed connection to the sensor. The oxygen sensor in the preferred embodiment of the invention supplies an electrical signal that toggles or changes state at the stoichiometric value of the air/fuel ratio. Malfunction at or a severed connection to the cable is sensed as a failure of the sensor output signal to toggle for a preselected time duration, which is selectively programmable by a user. In this event, a control signal is applied to the valve to place or "park" the valve at the most recent position at which the oxygen sensor toggled from lean to rich. At start-up, the valve is positioned or "parked" at a preselected percentage of a fully opened condition, which again is preferably programmable by a user.
In the preferred embodiment of the invention, several parameters of valve control are selectively programmable by a user, including rate of change of the control signal as the signal from the oxygen sensor varies or toggles around the stoichiometric value. Rate of change may be selectively programmed in the preferred embodiment of the invention at differing rates for when the signal from the oxygen sensor varies from rich to lean and from lean to rich. In this way, the system may be biased toward either rich or lean operation. Other valve control parameters that are selectively programmable by the user include speed of valve movement, valve damping, the maximum closed and minimum open positions, and filtration of the input signal from the engine sensor, which preferably comprises a manifold air pressure sensor. In accordance with another feature or aspect of the present invention, the electronic control unit is coupled to a status light on an operator panel, and is programmed to flash the status light at intervals and durations indicative of differing operating conditions at the engine. All features or aspects of the invention may be implemented separately from, or more preferably in combination with, each other.